The yield of hydrocarbons, such as gas and petroleum, from subterranean formations can be increased by fracturing the formation in order to stimulate the flow of these hydrocarbons in the formation. Various formation fracturing procedures are now used, such as, for example, hydraulic fracturing in which liquids, gases and or combinations of both are injected into the formation under high pressure (usually with propping agents).
Hydraulic fracturing is often used in the industry for improving oil and natural gas production from subterranean formations. During a hydraulic fracturing operation, a fluid, generally termed a “pad,” is pumped down a well at sufficient pressure to crack or fracture open the formation surrounding the well. Once a fracture has been created, the pumping of the pad, along with a slurry phase that comprises both the liquid and a proppant, is begun until a sufficient volume of the proppant has been carried by the slurry into the fracture. After a suitable time, the pumping operation is stopped at which time the proppant will prop open the fracture in the formation, thereby preventing it from closing. As a result of the fracture, trapped hydrocarbons are provided a more conductive pathway to the wellbore than was previously available, thereby increasing the well's production. In addition to creating deep-penetrating fractures, the fracturing process is useful in overcoming wellbore damage, to aid in secondary operations and to assist in the injection or disposal of produced formation brine water or industrial waste material.
During the fracturing process, the fractures propagate throughout the formation. The vertical propagation of these fractures is useful in determining the extent of fracture coverage as it relates to the producing interval. Fracture height measurements aid well operators in determining the success of the fracturing operation and, if necessary, to optimize future treatments for other wells in the field. In addition, fracture height information can aid in the diagnosis of stimulation problems such as lower production rates or unfavorable water cuts. The fracture height data can indicate whether communication has been established between the producing formation and adjacent water or non-hydrocarbon producing formation zones. Height measurements also provide a check on the accuracy of fracture design simulators used prior to the job to predict fracture geometry. If excessive fracture height growth is determined this would imply that the fracture length is shorter than the designed value.
As stated above, one reason for monitoring the vertical propagation of a fracture is the concern for fracturing outside of a defined hydrocarbon-producing zone into an adjacent water-producing zone. When this occurs, water will flow into the hydrocarbon-producing zone and the wellbore, resulting in a well that produces mainly water instead of the desired hydrocarbon. Furthermore, if there is still the desire to continue producing hydrocarbons from the well, operators must solve the serious problem of safely disposing of the undesired water. Addressing the problems arising from an out-of-zone fracture will also add expenses to the operation. In addition, if the fracture propagates into an adjacent non-hydrocarbon producing formation, the materials used to maintain a fracture after the fluid pressure has decreased may be wasted in areas outside the productive formation area. In short, it is expensive to efficiently operate a well that has been fractured out of the hydrocarbon-producing zone.
Because of the serious problems that can occur as a result of out-of-zone fractures, it is desirable to determine formation fracture development. Several techniques and devices are known for monitoring and evaluating formation fracture development, such as radioactive tracers in the fracturing fluid, temperature logs, borehole televiewers, passive acoustics and gamma-ray logging. Most techniques provide some direct estimates of fractured zone height at the wellbore.
One known process used to determine formation fracture height development employs a radioactive tracer. In this process, a fracturing fluid containing a radioactive tracer is injected into the formation to create and extend the fractures. When these radioactive fluid and proppant tracers are used, post fracture gamma-ray logs have shown higher levels of activity opposite where the tracer was deposited, thereby enabling operators to estimate the vertical development of the propped fractures.
Another approach for determining fracture height uses temperature and gamma-ray logs. Temperature logs made before and after stimulation are compared to define an interval cooled by injection of the fracturing fluid and thus provide an estimate of the fractured zone. However, this technique is subject to limitations and ambiguities. For example, the temperature log may be difficult to interpret because of low temperature contrast, flowback from the formation before and after the treatment, or fluid movement behind the borehole casing. In addition, the use of radioactive tracers may give rise to environmental problems such as the pollution of underground water streams, and the like, and hence be undesirable.
Other known methods for evaluating fracture geometry include using a borehole televiewer or using acoustical methods. Utilizing a borehole televiewer is limited in that it can only be used for fracture height evaluation in open holes. In addition, utilizing a borehole televiewer is limited due to the extreme temperature and pressure conditions present in deeper completions. Acoustical methods are hampered by inhomogeneous formation impedance and/or the need for pumping while the tool is in the wellbore.
In addition to the problems associated with each type of known monitoring method, there are inherent problems in the formation fracturing technology. During the fracturing process, fracture fluid is generally pumped into the formation at high pressure, to force open the fractures, and an increasing proportion of sand is added to the fluid to prop open the resulting fractures. One problem with the existing technology is that the methods for determining whether a formation has been fractured out of the production zone relies on post-treatment (i.e. after the fracture has occurred) measurements. In such systems, a fracturing treatment is performed, the treatment is stopped, the well is tested and the data is analyzed. Moreover, with existing detection systems, the wait for post-fracturing data can take a considerable amount of time, even up to several days, which can delay the completion operations, resulting in higher completion and operating costs.
Another problem associated with existing post-process “logging” or measuring devices is that the cost associated with interrupting a fracturing job in order to make a measurement of a fracture is neither practical nor feasible. Because the fracturing fluid is pumped into a formation under high pressures during the fracturing process, temporarily halting the pumping during the fracturing operation will result in the application of pressure to the fracturing fluid by the walls of the formation fracture. This could lead to undesirable results such as the closing of the fractures, thereby causing the reversal of fluid flow back into the borehole, or the build-up of sand in the hole. In addition, after taking measurements and completing the logging process, operators cannot restart the pumping equipment at the point of the fracturing process immediately before the interruption. Instead, the operators would have to repeat the complete fracturing job at additional cost and with unpredictable results.
A fracture monitoring system that does not require interrupting a fracturing job could address the above-described problems and would allow well operators to monitor the fracturing process, to control fracture dimensions and to efficiently place higher concentrations of proppants in a desired formation location. In addition, if there is information that a fracture is close to extending outside the desired zone, operators can terminate the fracturing job immediately. Furthermore, analysis of the ongoing treatment procedure will enable an operator to determine when it is necessary to pump greater concentrations of the proppant, depending on factors such as the vertical and lateral proximity of oil/water contacts with respect to the wellbore, the presence or absence of water-producing formations and horizontal changes in the physical properties of the reservoir rock.